1. Field of the Invention
The present invention relates to a compound semiconductor switching device for switching at high frequencies, especially to a compound semiconductor switching device used at a frequency of about 2.4 GHz or higher.
2. Discussion of the Related Art
Mobile communication devices such as mobile telephones often utilize microwaves in the GHz range, and commonly need a switching device for high frequency signals, which are used in switching circuits for changing antennas and switching circuits for transmitting and receiving such signals. A typical example of such a switching device can be found in Japanese laid-open patent publication No. Hei 9-181642. Such a device often uses a field-effect transistor (called FET hereinafter) made on a gallium arsenide (GaAs) substrate, as this material is suitable for use under high frequencies, and developments have been made in forming a monolithic microwave integrated circuit (MMIC) by integrating the aforementioned switching circuits.
FIG. 6(A) is a cross-sectional view of a conventional GaAs FET. The GaAs substrate 1 is initially without doping, and has, beneath its surface, an n-type channel region (or a channel layer) 2 formed by doping with n-type dopants. A gate electrode 3 is placed on the surface of the channel region 2 forming a Schottky contact, and two signal electrodes, namely a source electrode 4 and drain electrode 5, are placed on both sides of the gate electrode 3 forming ohmic contacts to the surface of the channel region 2. In this transistor configuration, a voltage applied to the gate electrode 3 creates a depletion layer within the channel region 2 beneath the gate electrode 3, and thus controls the channel current between the source electrode 4 and the drain electrode 5.
FIG. 6(B) shows the basic circuit configuration of a conventional compound semiconductor switching device, called a SPDT (Signal Pole Double Throw), using GaAs FETs. One of the two signal electrodes, which can be either a source electrode or a drain electrode, of each FET (FET1 being the first FET and FET2 being the second FET) is connected to a common electrode IN. Another of the two signal electrodes of each FET (FET1 and FET2) is connected to an output terminal OUT1 and OUT2). The gates of FET1 and FET2 are connected to the control terminals Ctl-1 and Ctl-2 through resistors R1 and R2, respectively. A complementary signal is applied to the first and second control terminals, Ctl-1 and Ctl-2. When a high level signal is applied to the control terminal of an FET, the FET changes into an on-state, and a signal passes from the common input terminal IN to the output terminal of the FET. The role of the resistors R1 and R2 is to prevent the leaking of the high frequency signals through the gate electrodes to the DC voltage applied on the control terminals (Ctl-1 and Ctl-2), which are grounded through capacitors.
An equivalent circuit of the aforementioned conventional compound semiconductor switching device is shown in FIG. 7. In microwave technology, the standard characteristic impedance is 50 Ω, and, thus, in this case the characteristic impedance of each terminal is 50 Ω (R1=R2=R3=50 Ω). With the voltages of the three terminals being represented by V1, V2, and V3, respectively, the insertion loss and the isolation are given by the following equations I and II:Insertion Loss=20 log(V2/V1)[dB]  [I]Isolation=20 log(V3/V1)[dB]  [II]
Equation I states the insertion loss expressed in dB when a signal is transmitted from the common input terminal IN to the output terminal OUT1, and equation II expresses the isolation between the common input terminal IN and the output terminal OUT2, also in dB.
In this type of compound semiconductor switching device, it is required that the insertion loss be minimized while improving the isolation, when one designs an FET which is inserted in series into the circuit. The reason a GaAs FET is used as the FET in this type of device is that GaAs has a higher electron mobility than silicon and thus a lower electrical resistance, making it easier to attain a low insertion loss, and that a GaAs substrate is a semi-insulating material suitable for attaining high isolation between the signal passes. On the other hand, GaAs substrates are more expensive than silicon substrates, and such devices cannot compete with silicon devices once a comparable device, such as a PIN diode, is made from silicon substrates.
In this type of compound semiconductor switching device, the electrical resistance R of the channel region 2 in the FET is given by the following expression in ohms:R=1/enμS[Ω]  [III]where e denotes the electric charge of an electron (1.6×10−19 C/cm3), n the electron carrier concentration, μ the electron mobility, and S the surface area of the cross section of the channel region (cm2).
As seen from equation III, the conventional guideline for designing such device is to maximize the gate width and thereby the cross-sectional area of the channel region, for reducing the electric resistance and thus the insertion loss.
However, the above configuration makes the capacitance of the Schottky contact between the gate 3 and the channel region 2 very large, and allows leakage of the high frequency input signal, resulting in a reduced degree of isolation. In the conventional design, shunt FETs are introduced into the circuit to prevent deterioration of the isolation.
FIG. 8 shows a circuit of a compound semiconductor switching device which has been used in commercial products. In this configuration, shunt FET3 and shunt FET4 are introduced between the output terminals OUT1, OUT2 of the switching FETs and the ground, such that the complementary signals from the control terminals Ctl-1 and Ctl-2 are applied on the gates of shunt FET3 and shunt FET4, respectively. As a result, when FET1 is in an on-state, shunt FET4 is also in an on-state while FET2 and shunt FET3 are in an off-state.
In this configuration, when the signal between the common input terminal IN and the output terminal OUT1 is switched on, and accordingly the signal between the common input terminal IN and the output terminal OUT2 is switched off, the input signal leaking to the output terminal OUT2 is directed to the ground through a capacitor C connected to the ground. Thus, it is possible to improve the isolation over the configuration without the shunt FETs.
In summary, the conventional design guideline for the compound semiconductor switching device is to increase the gate width in order to reduce the on-state resistance, thereby reducing the insertion loss. The large width of the gate electrode, however, leads to increased capacitance of the gate electrode, resulting in reduced isolation. Thus, it is inevitable that a shunt FET has to be introduced into the circuit for directing the leaking input signal to the ground for improving isolation.
Thus, the conventional compound semiconductor switching device has an extremely large chip size with the typical size being 1.07×0.50 mm2. This is against the current semiconductor design trend in which cost reduction is sought by reducing the chip size. As a result, expensive compound semiconductor devices for switching have been replaced by the inexpensive silicon-based counterparts and has lost its share in the market.